Colorectal cancer (CRC) is one of the most common cancers in both men and women in the United States, and comprises about 10% of new cancer cases. Colorectal cancer is an age-related epithelial cancer that often takes over twenty years to progress from initial benign adenoma (polyp) to invasive adenocarcinoma. The majority of colon cancers are characterized by genetic mutations in adenomatous polyposis coli (APC) or B-catenin, two key components of the Wnt signaling pathway. In the absence of Wnts, B-catenin is normally targeted for degradation by the GSK-3/Axin/APC complex. In the presence of Wnts, this degradation pathway is inhibited, B-catenin accumulates in the cytoplasm and nucleus and binds and transactivates Tcf/Lef proteins. Polyps arise when there is inappropriate activation of Wnt signaling, and B-catenin protein accumulates in the nucleus. We have recently developed a new mouse model of colon cancer where mice develop colon tumors that are consistent with those found in human FAP patients. As part of this proposal, we will further characterize this model using molecular tumor mariners. Additionally using mouse genetics, we will rigorously test via gain and loss of function studies, the role of Tcf4 on colon tumor formation in this mouse colon tumor model. Tcf4 is a member of the Tcf/Lef family of transcription factors that is essential for small intestinal cell proliferation and is expressed in human colon cancer cell lines and primary colon tumors. Taken together, results from these studies will provide important new insights into the role of Tcf4 during colon tumor formation, and may reveal novel downstream targets of Tcf4 function.